setup: raise minimum Python version to 2.7.4

On older Python versions, Mercurial is not really usable because of
https://bugs.python.org/issue10211.

Recently someone reported a crash on the mailing list when running Mercurial
on Python 2.7.3. There was consensus that fixing compatibility for a Python
version more than 7 years old is not worth it. So, instead of making Mercurial
crash with an obscure exception, this patch raises the minimum Python version
to 2.7.4.

setup.py

@@ -13,7 +13,7 b' import os'
 # bug link: https://bugs.python.org/issue25270
 supportedpy = ','.join(
     [
-        '>=2.7',
+        '>=2.7.4',
         '!=3.0.*',
         '!=3.1.*',
         '!=3.2.*',
@@ -54,7 +54,7 b' else:'
 # should have a chance of getting a 4.2 release, and when we ratchet
 # the version requirement forward again hopefully everyone will get
 # something that works for them.
-if sys.version_info < (2, 7, 0, 'final'):
+if sys.version_info < (2, 7, 4, 'final'):
     pip_message = (
         'This may be due to an out of date pip. '
         'Make sure you have pip >= 9.0.1.'
@@ -74,7 +74,7 b" if sys.version_info < (2, 7, 0, 'final')"
     except Exception:
         pass
     error = """
-Mercurial does not support Python older than 2.7.
+Mercurial does not support Python older than 2.7.4.
 Python {py} detected.
 {pip}
 """.format(